The main principle of MTO (methanol-to-olefins process), lies in that methanol is first produced from synthetic gas which is derived from coal used as the starting material, and then the methanol is converted to lower olefins (which mainly include ethylene and propylene) which are used for other chemicals and industrial articles afterwards. The MTO process flow primarily concerns a reaction unit, a product purifying unit and a product separating unit, wherein the reaction unit comprises a reactor and a regenerator, the product purifying unit comprises a quench tower, a water scrubbing tower, an alkaline washing tower and a drying tower, and the remaining part is the product separating unit. The reactor generally used in the MTO process is a fluid-bed reactor in which methanol, as the reactant, and olefins, as the products, are present in gas phase unexceptionally. The ratio of ethylene to propylene varies depending on the specific process applied. Current representative MTO arts include the processes developed by UOP, UOP/Hydro, Exxon Mobil and Dalian Chemical and Physical Research Institute of PRC. Along with the development of national economy and the growing demand on lower olefins, the starting materials of ethylene such as naphtha and light diesel oil are increasingly short of supply. Therefore, the concern of speeding up the industrial application of a MTO process has gained attention from various quarters.
In a MTO process, the product discharge stream that comes from the reactor and carries catalyst particles flows into a quench tower to be quenched. In the quench tower, water and methanol are condensed into liquids, while the product of olefins is still present in gaseous state. Since most of the catalyst particles will be captured by the quench water and get discharged from the quench tower, the quench water is rich in catalyst particles. In order to further remove the catalyst powder entrapped in the product gas stream, the gaseous olefin stream is fed into a water scrubbing tower to completely remove the catalyst via scrubbing. As a result, the catalyst powders are entrapped in the scrubbing water. Part of the quench water is cooled and then returns to the quench tower to be used as quench agent again, while the other part is fed into a stripping tower to separate methanol from water via stripping. The heat exchanger is so easily blocked by the large amount of catalyst particles in the quench water that the quench water must be purified. On the other hand, the catalyst particles in the stripping tower will deposit at the bottom of the tower, so that it is necessary to further remove the catalyst from the quench water before it enters the stripping tower in order to ensure the long-cycle operation of the stripping tower and reduce the pollution of downstream materials by the catalyst particles. If the catalyst in the scrubbing water is not removed, it will continuously accumulate, and therefore deteriorate the scrubbing efficiency as well as imperil the normal operation of the heat exchanger which is used to cool the scrubbing water before it is recycled.
U.S. Pat. No. 6,166,282 discloses a MTO fluid bed reactor, on top of which is equipped a set of cyclones to separate the solid catalyst particles from the product discharge stream, in order to prevent the catalyst particles from being carried out of the reactor. Despite the fact that the reaction gas stream flows through several cyclones, there are still catalyst particles which will exit the reactor along with the product discharge stream. These catalyst particles are separated from the gaseous product along with the quench water and the scrubbing water respectively in the quench tower and the scrubbing tower, and enter the water recycling system, leading to abrasion and clogging of the downstream equipments as well as the reduction of the operating cycle of the installation.
U.S. Pat. No. 5,744,680A discloses a method for preparing light hydrocarbons from oxides, wherein the stream comprising catalyst is expelled in a wet washing step. U.S. Pat. No. 6,870,072 also discloses that a wet washing section is used to remove the catalyst from the product discharge stream.
With respect to MTO quench water, U.S. Patent Application Publication No. 2005/0234281 A1 and Chinese Patent No. 1,942,558A disclose a combination of one or more solid-liquid cyclones or hydrocyclones operating in sequence or parallel for separating the catalyst which is recycled thereafter. The product stream has passed through several gas-solid cyclone separating units before it is discharged from the reactor, thus the catalyst particles in the MTO quench water have very small particle size (1-20 μm, wherein 80% of them have a particle size of less than 10 μm), with those in the scrubbing water even smaller (less than 5 μm). However, provided with an effective capture size in the range of 5-10 μm, a conventional solid-liquid cyclone or hydrocyclone can only be used to remove the catalyst particles lost under such abnormal conditions as that the solid-liquid cyclone does not function or the operation fails. In other words, solid particles smaller than 2 μm are hard or even impossible to be removed when the cyclone works normally. Thus, a conventional solid-liquid cyclone or hydrocyclone can only be used in an accident when catalyst particles are lost, whereas it can not fulfill the expected effect under normal operating conditions.
In addition, although one or more gas-solid separating devices are installed on top of a fluid bed reactor in prior art, a small amount of fine catalyst particles are still entrapped in the gaseous product stream exiting the reactor. The catalyst particles are expelled from the quench tower along with the reaction effluent and result in the containment of solid particles, which are hard to be removed, in the quench water to be stripped.
Therefore, taking into account the characteristics of the catalysts contained in the quench water and the scrubbing water of a MTO process as well as the operating conditions (i.e. the even distribution, small content and large throughput of the catalyst particles), it is highly desired in the art to develop a method for removing solid particles from MTO quench water and scrubbing water via mini-hydrocyclone separation under normal operating conditions.